wait for the scritp console

The script console is added by Javascript and not completely ready with
the page load.
Therefore we need to wait for the parts we want to manipulate to be
ready.

Fixes a flaky test observed when using master.runStript() with the following failure:

```
org.openqa.selenium.NoSuchElementException:
Unable to locate element: //*[@path='/script']

```

src/main/java/org/jenkinsci/test/acceptance/po/CodeMirror.java

@@ -24,6 +24,7 @@
 package org.jenkinsci.test.acceptance.po;
 
 import org.openqa.selenium.JavascriptExecutor;
+import org.openqa.selenium.NoSuchElementException;
 
 /**
  * Encapsulate CodeMirror wizardry.
@@ -47,7 +48,8 @@ public void set(String content) {
         }
 
         // can't use find() because it wants a visible element
-        driver.findElement(by.xpath("//*[@path='%s']", getPath()));    // wait until the element in question appears in DOM
+        // wait until the element in question appears in DOM as it is added by JavaScript
+        waitFor().ignoring(NoSuchElementException.class).until(() -> driver.findElement(by.xpath("//*[@path='%s']", getPath())));
 
         executeScript(scriptSet, String.format("//*[@path='%s']/following-sibling::div", getPath()), content);
     }